Method and apparatus for attenuating sound waves in gas streams



Nov. 23, 1965 F. E. DEREMER 3,219,142

METHOD AND APPARATUS FOR ATTENUATING SOUND WAVES IN GAS STREAMS Filed Oct. 3, 1962 2 Sheets-Sheet l ILZZTIJ i 1 til: ::I i

IN V EN TORI FL 0207 E. .UEREMER.

ATTORNEY 1965 F. E. DEREMER METHOD AND APPARATUS FOR ATTENUATING SOUND WAVES IN GAS STREAMS Filed Oct. 5, 1962 2 Sheets-Sheet 2 INVENTOR: FL UYlJ E. DEREMER. BY

ATTORNEY United States Patent f 3,219,142 METHOD AND APPARATUS FOR ATTENUATING SOUND WAVES IN GAS STREAMS Floyd E. Dercmer, Livonia, Mich., assignor t0 Oldberg Manufacturing Company, Grand Haven, Mich., a corporation of Michigan Filed Oct. 3, 1962, Ser. No. 228,131 2 Claims. (Cl. 18148) This invention relates to a method, system and apparatus for attenuating sound waves in gas streams and more especially to a method of and apparatus for attenuating sound waves in exhaust gas streams from an internal combustion engine of an automotive vehicle.

It has been conventional practice in automotive vehicles powder by internal combustion engines to convey exhaust gases from the engine exhaust manifold or manifolds through a sound attenuating mufiler or silencer and the exhaust gases conveyed from the mufller through a tail pipe rearwardly of the vericle. The mufllers or silencers heretofore used are usually of from two to four feet in length depending upon the interior gas passage and sound attenuating construction of the muffler unit. The efficiency of sound attenuation of sound waves in an exhaust gas stream is dependent in a large measure upon the provision of sound attenuating means of a character which will attenuate or dampen sound waves of various lengths, some low frequency sound waves being of a length of several feet.

Mufflers or silencers have been constructed of substantial length to facilitate straight through flow or passage of the gases through the muffler and such installations were used in vehicles where a space of sutficient length was available to accommodate such installations. The trend toward lowering the vehicle silhouette and the use of multiple frame or transverse bracing means or members beneath the vehicle floor necessitated shortening of the mufiler or silencer units. In order to obtain a reasonable length of gas travel for sound attenuating purposes, muflle-r units have been made having internal gas passage tubes arranged to reverse the travel of the gas through the mufiler in a S-shaped path, a construction which necessarily increases exhaust gas back pressure as well as entailing difiiculties in cooling and rapid deterioration of the gas passage means. Such muffler units in many instances provide an insufficient length of gas travel for proper sound attenuation and hence the use of supplemental mufflers or resonators have been employed in conjunction with primary muffler units and spaced therefrom in an endeavor to secure more efiicient sound attenuation. Such arrangements or installations are costly and fail to satisfactorily attenuate many of the longer sound waves in the gas streams.

The present invention embraces a method or system providing for greatly increased length of gas passage means associated with sound attenuating instrumentalities whereby to obtain more efficient sound attenuation and particularly attenuation of sound waves of substantial lengths and effect a substantial reduction in back pressure.

An object of the invention is the provision of a method of or system for improving sound attenuation in an eX- haust gas stream of an automotive vehicle wherein the gases move generally in one direction through lengthwise 3,219,142 Patented Nov. 23, 1965 spaced sound attenuating media with a minimum of exhaust gas back pressure and providing for improved sound attenuation.

Another object of the invention is the provision of a sound attenuating means particularly for attenuating sound waves in an exhaust gas stream from an internal combustion engine in an automotive vehicle wherein the gases are conveyed from an exhaust manifold through a single tubular means to the rear of the vehicle with resonators or sound attenuating means spaced lengthwise within the tubular means whereby sound waves of varying lengths may be satisfactorily attenuated.

Another object of the invention resides in the provision of a combined exhaust pipe and sound attenuating media facilitating conveyance of exhaust gases generally in one direction thereby eliminating reverse flow of gases whereby deposits of particulate material resulting from combustion entrained in the exhaust gases are substantially eliminated.

Another object of the invention is the provision of a single tubular means or pipe for conveying an exhaust gas stream away from an internal combustion engine wherein a plurality of sound attenuating units or resonators are arranged in spaced relation for attenuating sound waves entrained in the gas stream, the spacing between individual resonators or sound attenuating means being such that objectionable sound waves may be tuned out, attenuated or absorbed.

A further object of the invention resides in the provision of a tuned exhaust pipe or system comprising a combined exhaust pipe and sound attenuating means or media of substantial length embodying spaced resonators within the pipe, the pipe being of substantial length whereby pressure impulses in the gas stream are more effectively equalized thereby minimizing the throbbing usually attendant the discharge of exhaust gases from a conventional muffler and exhaust pipe.

A further object of the invention resides in the provision of a single exhaust conveying tube of substantial length embodying spaced sound attenuating devices or resonators arranged whereby the pipe may be configurated to avoid interference with structural components of the vehicle yet providing for movement of the gases generally in one direction from the exhaust manifold of the engine for discharge rearwardly of the vehicle.

Further objects and advantages are within the scope of this invention such as relate to the arrangement, operation and function of the related elements of the structure, to various details of construction and to combinations of parts, elements per se, and to economics of manufacture and numerous other features as will be apparent from a consideration of the specification and drawing of a form of the invention, which may be preferred, in which:

FIGURE 1 is a phantom or schematic view of a unitized type of frame construction of an automotive vehicle embodying an internal combustion engine and illustrating a form of tuned exhaust pipe or sound attenuating system of the invention;

FIGURE 2 is an elevational view of the sound attenuating or tuned exhaust pipe or tube illustrated in FIGURE 1 showing the sound attenuating media or resonators in spaced relation;

FIGURE 3 is an elevational view of an exhaust pipe embodying spaced resonators illustrating one method of configurating or bending a straight pipe at regions spaced from the resonators to avoid interference with frame or structural components of the vehicle with which the pipe may be used;

FIGURE 4 is a longitudinal sectional view illustrating one form of combined exhaust pipe and resonator or sound attenuating media comprising a series of sound attenuating units in end-to-end relation and welded to provide a continuous tubular construction;

FIGURE 5 is an enlarged sectional view taken substantially on the line 55 of FIGURE 4;

FIGURE 6 illustrates another method of forming a continuous tuned exhaust utilizing tubular couplings between adjacent sound attenuating units;

FIGURE 7 is a longitudinal sectional view illustrating a one piece tubular exhaust pipe embodying spaced sound attenuators or resonators wherein regions of the exhaust pipe are distorted or spun inwardly to secure the resonators in proper position in the exhaust pipe;

FIGURE 8 is an elevational view of tuned exhaust pipe embodying sound attenuator or resonator units telescoped in the exhaust pipe;

FIGURE 9 is an enlarged transverse sectional View taken substantially on the line 99 of FIGURE 8, and

FIGURE 10 is a sectional view illustrating an exhaust pipe embodying another form of resonator or sound attenuating means.

The combined exhaust pipe and sound attenuator construction or tuned exhaust tube has particular utility with vehicle constructions powered by internal combustion engines as the arrangement of the invention provides effective length for attenuation of sound waves of varying lengths, the tube being configurated to avoid interference with structural components of the vehicle. It is to be understood that the method and arrangement of sound attenuation and gas conveying means of the invention may be used wherever it may be found to have utility.

Referring particularly to FIGURE 1, there is illustrated in phantom or schematic form a vehicle frame 10 of socalled unitized construction preferably formed of sheet metal shaped to the desired configuration by suitable dies, the frame construction embodying a forward panel or engine mounting portion 12, an intermediate raised portion 14 to accommodate power transmission means and a transversely raised portion 16 to accommodate vertical movements of the rear axle or axles of the vehicle. The body construction 10 is fashioned with lengthwise extending side flanges 18 to lend rigidity or stiffness to the frame contruction and transversely arranged reinforcing members.

The forward portion 12 of the frame construction supports an internal combustion engine 20 which, in the embodiment illustrated, is of the V-type construction embodying two banks of cylinders indicated at 22 and 24. The bank of cylinders 22 is provided with an exhaust manifold 26 having an exhaust pipe connection 28. The bank of cylinders 24 is provided with an exhaust manifold 30 fashioned with an exhaust pipe connection 32. In the embodiment illustrated, the exhaust pipe connections 28 and 32 are joined respectively with portions 34 and 36 of a dual or Siamese pipe or coupling construction 38.

In the arrangement shown in FIGURE 1, the gas streams from both banks of cylinders are converged into a single combined exhaust pipe and sound attenuating construction or tuned exhaust pipe 40. The exhaust pipe construction extends to a region at the rear of the vehicle and the exhaust gases are discharged from the distal end 41 of the tuned exhaust pipe.

In the embodiment illustrated in FIGURE 1, the exhaust gas streams from both banks of cylinders are combined for conveyance through the single tuned exhaust pipe 40. It is to be understood however that a separate tuned exhaust pipe may be employed for each bank of cylinders if desired. The use of a single exhaust pipe 40 or two exhaust pipes of substantially identical character, one for each bank of cylinders, is dependent upon the volume of exhaust gases to be conveyed through the tuned exhaust pipe construction.

The exhaust pipe arrangement may be a single full length tube or an assembly of individual lengths of tubing welded or otherwise secured together as hereinafter described. The pipe 40, as shown in FIGURES 1 through 3, embodies three sound attenuators or resonator units 42, 44 and 46 arranged in lengthwise spaced relation, the relative positions of the units depending upon the available space adjacent the vehicle frame and body to accommodate the sound attenuating units and the particular positioning of the sound attenuating units to obtain the desired sound attenuation or sound absorption.

In the tuned exhaust pipe construction shown in FIG- URES 1 through 3, the exhaust pipe has a bent region at 50 between adjacent resonators 42 and 44 and a second bent region adjacent the raised portion 16 of the vehicle frame to avoid interference with the vertical movement of the rear axle or axles of a vehicle. The bent zones at the regions 50 and 52 may be fashioned in the exhaust pipe or tube by means of a suitable bending tool 54 or the bends may be fashioned in the tube by dies through the use of a press.

The exhaust gases enter the forward end of the tube 40 and travel in a rearward direction, the only deviation in rectilinear travel being the path of traverse at the bends 5t) and 52 or other bent regions that may be fashioned in the exhaust pipe and through sound attenuation chambers or regions adjacent gas passage means in the sound attenuating units hereinafter described.

It is to be understood that while two bent regions 50 and 52 are illustrated in FIGURES 1 through 3, other bends may be fashioned in the pipe to avoid interference with components of the vehicle frame or body construction. The resonators or sound attenuating units 42, 44 and 46 are preferably disposed in rectilinear regions of the exhaust pipe and may be spaced as desired to secure most effective sound attenuation characteristics or tuning as the bent regions which are not readily susceptible for accommodating the sound attenuators. It is to be understood that the sound attenuators or attenuatining units may be of varying lengths depending upon the character of sound wave attenuation or sound wave damping desired.

FIGURE 4 is illustrative of a tuned exhaust pipe or combined exhaust pipe and sound attenuator construction 56 which is made up of several resonator units of a particular type of construction, the ends of the units being welded together, either in abutted relation or in overlapping telescoping relation, so that the final assembly is a continuous tubular means with spaced sound attenuators or attenuating units. The pipe 56 has four sound attenuating units designated respectievly 57, 58, 59 and 60 which may be in spaced relation, the spaces between adjacent units being dependent upon the length required for bends or distorted regions in the pipe.

The sound attenuating unit 57, in the form illustrated, comprises a central tubular member or inner shell 62 forming a gas passage means, the wall of which is provided with a comparatively large number of small openings forming acoustic couplings 64.

The outer tube, shell or tubular portion 66 of the attenuating unit 57 is distorted or spun inwardly at peripheral regions 68 to engage inwardly extending annular ridges thus formed on the shell with the exterior surface of the inner shell or tubular member 62. Through this arrangement sound attenuating chambers 70 and 72 are formed which are acoustically coupled with the inner shell 62 through the small openings 64, the annular chambers or regions 70 and 72 functioning to attenuate sound waves.

The sound attenuating unit 57 may be butt welded to the adjacent resonator 58 at the region indicated at 74 to form a continuation of the outer tube 66 with the 3 outer tube of the sound attenuating unit 58. If desired, one end region of the outer tube of an attenuating unit may be of slightly larger diameter to be telescoped with the end region of the adjacent unit and the overlapping regions welded or sealed in a suitable manner to form a continuous imperforate outer shell or exhaust pipe.

The sound attenuating unit or means 58 may be formed with an inner shell or inner gas passage tube 78 and the outer tube or shell 76 may be spun or distorted inwardly at lengthwise spaced regions designated 80, 81 and 82 into engagement with the outer surface of the inner shell 78 to provide sound attenuating chambers 84 and 86.

The inner shell 78 is acoustically coupled with the annular chambers or regions 84 and 86 through a comparatively large number of small openings 88 formed in the wall of the inner shell 78. The sound attenuating chambers 84 and 86 may be of varying lengths depending upon the spacing of the inwardly extending peripheral regions 80, 81 and 82 of the outer shell or tube 76.

The sound attenuating unit 58 may be coupled with the adjacent unit 59 by butt welding at the region indicated at 90. The sound attenuating unit 59, in the illustration of FIGURE 4, includes an inner shell or tube 92 which is displaced centrally of the outer shell or tube 94 by the inwardly spun or distorted regions 96 and 98 to form comparatively long sound attenuating chamber 100 of a length defined by the inwardly extending portions 96 and 98. The inner tube or gas passage means 92 is acoustically coupled with the chamber 100 through a comparatively large number of small openings 102 formed in the wall of the inner shell 92.

In the unit illustrated at 59 a single sound attenuating chamber 100 is provided for attenuating sound Waves. The sound attenuating units may be provided with restriction means to promote increased flow of exhaust gases through the annular sound attenuating chambers if desired. In the embodiment illustrated in FIGURE 4, the sound attenuating unit 59 is provided with a member or restrictor 104 disposed in the inner shell 92. The restrictor 104 may be in the form of an annularly shaped member or a short length tube preferably having a tapered portion 105 which defines a restricted passage in the restrictor 104.

The restrictor promotes increased flow of exhaust gases through the openings 102 at the left side of the restrictor, as viewed in FIGURE 4, for traverse through the chamber 100, such gases reentering the inner shell 92 through the openings 102 at the right side of the restrictor. It is to be understood that in the absence of a restrictor some gas flow takes place through the sound attenuating chambers surrounding the inner shells or gas passage tubes of the attenuators and such flow of exhaust gases through the sound attenuating chambers tends to equalize the term peratures and to reduce the temperatures by reason of the heat radiated or conducted from the gases through the outer walls of the units.

The outer tube or shell 94 of the sound attenuating unit 59 may be welded to the outer tube 108 of the adjacent unit 60 at the region indicated at 110. In the embodiment illustrated in FIGURE 4, the sound attenuating means or unit 60 includes an inner shell or gas passage tube 112 which is centrally spaced in the outer tube 108 by inwardly extending regions or ridge portions 114 and 116 formed in the outer tube 108. The region between the portions 114 and 116 and between the inner shell 112 and the tube 108 provides a sound attenuating chamber 118 of substantial length.

In the embodiment illustrated the wall of the inner shell 112 may be provided with comparativley few openings 120 of larger size to facilitate increased flow of exhaust gases through the chamber 118, the openings 120 forming acoustic couplings with the chamber 118.

The exhaust gases may be discharged from the distal end 122 of the sound attenuating unit 60 or may be conveyed away by a tail pipe (not shown) connected with the end of the unit 60.

While the various forms of sound attenuating unit, illustrated in FIGURE 4, are arranged in rectilinear alignment, it is to be understood that the regions between adjacent resonators may be of substantial length and such regions may be provided with bends as shown in FIGURE 2 to avoid interference with components of the vehicle frame construction. While the sound attenuating units are illustrated in FIGURE 4 as directly coupled together by welding as hereinbefore described, it is to be understood that the units may be spaced apart by tubular inserts or couplings of substantially the same diameter as the outer tube components and such inserts may be bent or configurated to avoid interference with frame components of the vehicle construction.

A construction of this character is illustrated in FIG- URE 6. In this figure a sound attenuating unit 124, which may be of any of the types herein shown or described, is interiorly telescoped into the end region of a tubular coupling member or insert 126 employed for coupling two attenuating units together. As shown in FIGURE 6 the coupling member or insert 126 may be bent or configurated to a desired shape to avoid interference with components of the vehicle frame construction.

The overlapping region 128 of the sound attenuating unit 124 and the coupling member or insert 126 may be welded, crimped or otherwise processed to form a sealed joint between the sound attenuator unit 124 and the coupling member 126.

Another form of combined exhaust pipe and sound attenuating construction or tuned exhaust pipe is illus trated in FIGURE 7. In this form, the exhaust pipe, outer tube or shell 130 may be a continuous tube and disposed within the tube 130 in spaced relation are inner shells 132, 134 and 136. An additional number of attenuating units may be provided within the tube. The inner shells may be of varying lengths depending upon the sound attenuating characteristics desired and the available rectilinear space beneath the vehicle frame or body construction.

The outer tube 130 may be distorted or spun inwardly at spaced regions into engagement with the adjacent inner shell to form sound attenuating chambers. In the em bodiment illustrated, the tube 130 is distorted inwardly at regions illustrated 138, 139 and 140 providing with the inner and outer tubular components, partitions defining sound attenuating chambers 142 and 144. The chambers 142 and 144 are acoustically coupled with the inner shell 132 through a plurality of small openings 146.

The outer tube 130 may be distorted or spun inwardly at may be spun or distorted inwardly at regions designated 148, 149 and 150 providing partitions forming sound attenuating chambers 152 and 154 which are acoustically coupled with the inner shell through a large number of small openings 156 formed in the wall of the inner shell 134.

The inner shell 136 may be secured in its central position within the tube 130 by spinning or distorting the tube inwardly at the region indicated at 160 and additional regions spaced therefrom (not shown) to form a third sound attenuating unit.

While the regions between adjacent attenuators illustrated in FIGURE 7 are of rectilinear shape, it is to be understood that these regions may be bent or distorted as hereinbefore described to avoid interference with frame components of the vehicle. In this form of construction, the outer tube 130 is a single component and is imperforate throughout its length. The arrangement illustrated in FIGURE 7 is adapted to attenuate sound waves in the same manner as in the other forms of the invention hereinbefore described.

Another form of combined exhaust pipe and sound at tenuating means or tuned exhaust tube is illustrated in FIGURES 8 and 9. In this form an exhaust pipe or tube 166 may be a single component and individual sound attenuating units of any of the forms illustrated in FIG- URES 4 or 10 may be inserted in the tube 166 and spaced lengthwise therein as desired to secure desired sound attenuating characteristics. In the form shown in FIG- URE 8, three sound attenuating units designated 168, 170 and 172 are arranged in spaced relation, the spacing depending upon the regions of the tube 166 to be bent or distorted to avoid interference with frame components of the vehicle.

The sound attenuating unit 168 may be of the character illustrated at 57 in FIGURE 4 comprising an outer shell 174, an inner shell 176, and spaced peripheral regions 178, 179 and 180 distorted or spun inwardly into engagement with the inner shell 176 providing partitions defining sound attenuating chambers 182 and 184, which are acoustically coupled with the interior of the inner shell 176 through a plurality of openings 1% formed in the wall thereof. The other attenuating units 170 and 172 may be of the same or similar construction or may be of any of the other constructions herein disclosed and described. Each of the attenuating units may be spot Welded at regions indicated at 192 to assure retention of the units in proper spaced relation in the exhaust pipe or outer tube 166.

Another form of sound attenuating unit, that may be used in conjunction with the same or similar attenuating units or which may be inserted in an exhaust pipe in the manner illustrated in FIGURE 8, is shown in FIGURE 10. In this form the outer tube or shell 2% encloses an inner shell or gas passage tube 202 which is of lesser diameter than the outer shell. Spaced lengthwise of the inner shell 262 and disposed between the inner shell and the outer tube 200 is a plurality of annular members or rings 2% which form partitions defining sound attenuating chambers 206. The sound attenuating chambers 206 are acoustically coupled with the interior of the inner shell 262 by means of a comparatively large number of small openings 203 in the wall of the inner shell.

It is to be understood that in all forms of the invention, the number and size of the openings in the walls of the inner shells providing acoustic couplings between the inner shells and the sound attenuating chambers are such as to secure most effective sound attenuation as well as to promote the flow of exhaust gases through the acoustic couplings and through the sound attenuating chambers.

It is to be understood that the number of attenuators or sound attenuating units in a tuned exhaust pipe or system and the spacing of such units is dependent in a large measure upon the lengths of sound waves to be attenuated. The number of sound attenuating units and the spacing between adjacent units is also dependent upon the spacing of bends or distorted regions of the exhaust pipe or tubular components between adjacent sound attenuating units to avoid interference with frame or struc tural components of the vehicle.

It will be apparent that the tuned exhaust pipe system or method of the invention provides substantial length for sound attenuation so that sound waves of considerable lengths as well as extremely short sound waves may be satisfactorily attenuated with a minimum of back pressure especially as the exhaust gases travels in a generally rectilinear direction and are diverted from such direction only at the regions of traverse of the gases through the bent or distorted portions or couplings in the exhaust system and the flow through the sound attenuating chambers.

It is further to be understood that the limitations upon the length of each individual sound attenuating unit is only restricted by the necessary bent or distorted regions fashioned in the exhaust tube to avoid interference with structural components of the vehicle. Through the arrangement of my invention, greatly improved sound attenuation has been attained and particularly of the longer wave lengths by reason of the substantial path of traverse of the stream of gases through attenuating media from the exhaust manifold of the engine to a discharge region at the rear of the vehicle. Through such arrangement the outer tube or exhaust pipe offers substantial heat transfer area so that the gases are more quickly reduced to lower temperatures than has heretofore been possible with conventional muffler constructions. Furthermore the continuous flow of the gases generally in one direction reduces the tendency for carbon particles to be deposited in the attenuating units.

It is apparent that, within the scope of the invention, modifications and different arrangements may be made other than as herein disclosed, and the present disclosure is illustrative merely, the invention comprehending all variations thereof.

I claim:

1. A cylindrical exhaust pipe for attachment directly to the exhaust mainfold of a vehicle engine:

(a) said exhaust pipe being of substantial length and having a generally continuous outer surface,

(b) at least two spaced sound attenuating tubes coaxially mounted within said pipe,

(0) said tubes being cylindrical and open ended and having a uniform outer diameter throughout their length,

(d) one of said tubes having a restriction on the inside thereof,

(e) another of said tubes having a uniform unrestricted inner diameter,

(f) each of said tubes being supported adjacent their ends by imperforate annular rings,

(g) said pipe having annular indentations of U-shaped cross section forming said annular support rings,

(h) said annular rings, the outer surfaces of said tubes between said rings, and the inner surfaces of said pipe between rings, forming annular chambers,

(i) said tubes having perforations in the outer surfaces of said tubes between said rings opening into said chambers,

(j) said tubes being straight throughout their length,

and

(k) said pipe in the area of said tubes being straight.

2. A cylindrical exhaust pipe for attachment directly to the exhaust manifold of a vehicle engine:

(a) said exhaust pipe having a substantially uniform diameter from front to rear and having a generally continuous and unobstructed outer surface,

(b) said exhaust pipe being of substantial length,

(0) at least two spaced sound attenuating tubes coaxially within said pipe,

(d) said tubes being cylindrical and open ended and having a uniform outer diameter throughout their length,

(e) one or said tubes having a restriction on the inside thereof,

(f) another of said tubes having a uniform unrestricted inner diameter,

(g) each of said tubes being supported adjacent their ends by imperforate annular rings,

(h) said annular rings, the outer surfaces of said tubes between said rings, and the inner surfaces of said pipe between said rings forming annular chambers,

(i) said tubes having perforations, in the outer surfaces of said tubes between said rings, opening into said chambers,

(j) said tubes being straight throughout their length,

and

(k) said pipe in the area of said tubes being straight.

(References on following page) References Cited by the Examiner UNITED STATES PATENTS Starkweather et a1. 181-48 Haas 18161 Cary et a1. 18148 Buck 181-48 X Rainville 181-70 Cooper 181-70 Johnson 18141 Ludlow et a1.

10 3,104,736 9/1963 Ludlow. 3,105,570 10/1963 Bezemes. 3,118,517 '1/1964 -Lud1ow 18159 5 FOREIGN PATENTS 897,025 11/1953 Germany. 1,052,173 3/1959 Germany.

415,446 8/1934 Great Britain.

10 LEO SMILOW, Primary Examiner.

C. W. ROBINSON, Examiner. 

2. A CYLINDRICAL EXHAUST PIPE FOR ATTACHMENT DIRECTLY TO THE EXHAUST MANIFOLD OF A VEHICLE ENGINE: (A) SAID EXHAUST PIPE HAVING A SUBSTANTIALLY UNIFORM DIAMETER FROM FRONT TO REAR AND HAVING A GENERALLY CONTINUOUS AND UNOBSTRUCTED OUTER SURFACE, (B) SAID EXHAUST PIPE BEING OF SUBSTANTIALLY LENGTH, (C) AT LEAST TWO SPACED SOUND ATTENUATING TUBES COAXIALLY WITHIN SAID PIPE, (D) SAID TUBES BEING CYLINDRICAL AND OPEN ENDED AND HAVING A UNIFORM OUTER DIAMETER THROUGHOUT THEIR LENGTH, (E) ONE OR SAID TUBES HAVING A RESTRICTION ON THE INSIDE THEREOF, (F) ANOTHER OF SAID TUBES HAVING A UNIFORM UNRESTRICTED INNER DIAMETER, (G) EACH OF SAID TUBES BEING SUPPORTED ADJACENT THEIR ENDS BY IMPERFORATE ANNULAR RINGS, (H) SAID ANNULAR RINGS, THE OUTER SURFACES OF SAID TUBES BETWEEN SAID RINGS, AND THE INNER SURFACES OF SAID PIPE BETWEEN SAID RINGS FORMING ANNULAR CHAMBERS, (I) SAID TUBES HAVING PERFORATIONS, IN THE OUTER SURFACES OF SAID TUBES BETWEEN SAID RINGS, OPENING INTO SAID CHAMBERS, (J) SAID TUBES BEING STRAIGHT THROUGHOUT THEIR LENGTH, AND (K) SAID PIPE IN THE AREA OF SAID TUBES BEING STRAIGHT. 